Cysteine or thiol proteases contain a cysteine residue at the active site responsible for proteolysis. Since cysteine proteases have been implicated in a number of diseases, including arthritis, muscular dystrophy, inflammation, tumor invasion, glomerulonephritis, malaria, and other parasite-borne infections, methods for selectively and irreversibly inactivating them provide opportunities for new drug candidates. See, for example, Meijers, M. H. M. et al., Agents Actions (1993), 39 (Special Conference Issue), C219; Machleidt, W. et al, Fibrinolysis (1992), 6 Suppl. 4, 125; Sloane, B. F. et al., Biomed. Biochim. Acta (1991), 50, 549; Duffy, M. J., Clin. Exp. Metastasis (1992), 10, 145; Rosenthal, P. J., Wollish, W. S., Palmer, J. T., Rasnick, D., J. Clin. Investigations (1991), 88, 1467; Baricos, W. H. et al, Arch. Biochem. Biophys. (1991), 288, 468; Thornberry, N. A. et al., Nature (1992), 356, 768.
Low molecular weight inhibitors of cysteine proteases have been described by Rich, Proteinase Inhibitors (Chapter 4, "Inhibitors of Cysteine Proteinases"), Elsevier Science Publishers (1986). Such inhibitors include peptide aldehydes, which form hemithioacetals with the cysteine of the protease active site. The disadvantage of aldehydes is their in vivo and chemical instabilities.
Aldehydes have been transformed into .alpha.,.beta.-unsaturated esters and sulfones by means of the Wadsworth-Emmons-Horner modification of the Wittig reaction (Wadsworth, W. S. and Emmons, W. D. (J. Am. Chem. Soc. (1961), 83, 1733: Equation 1).